The present invention generally relates to perpendicular magnetic recording mediums, and more particularly to a perpendicular magnetic recording medium having satisfactory perpendicular magnetic recording and reproducing characteristics.
Generally, when recording and reproducing a signal on and from a magnetic recording medium by use of a magnetic head, the magnetic head magnetizes a magnetic layer of the magnetic recording medium in a longitudinal direction of the magnetic recording medium (that is, in an in-plane direction) at the time of the recording and picks up the recording at the time of the reproduction. However, according to such a longitudinal magnetic recording system, it is known that the demagnetization field becomes high as the recording density increases and the demagnetization field introduces undesirable effects to the high density magnetic recording. Hence, in order to eliminate the undesirable effects of the demagnetization, a perpendicular magnetic recording system has been proposed in which the magnetic head magnetizes the magnetic layer of the magnetic recording medium in a direction perpendicular to the magnetic layer. According to the perpendicular magnetic recording system, the demagnetization field becomes low as the magnetic recording density increases, and theoretically, it is possible to realize a satisfactory high density magnetic recording in which there is no decrease in the remanent magnetization.
As a conventional perpendicular magnetic recording medium which is used in the perpendicular magnetic recording system, there is a perpendicular magnetic recording medium having a cobalt-chromium (Co-Cr) film formed on a base film by a sputtering process. As is well known, the Co-Cr film is extremely suited for use in the perpendicular magnetic recording medium because the Co-Cr film has a relatively high saturation magnetization (Ms) and favors magnetization in a direction perpendicular to the Co-Cr film (that is, the coercivity in the direction perpendicular to the Co-Cr film is large and the axis of easy magnetization is perpendicular to the Co-Cr film).
However, due to the addition of chromium (Cr), cobalt (Co) has such an orientation that the axis of easy magnetization thereof is approximately perpendicular to the Co-Cr film but the axis of easy magnetization is not perfectly perpendicular, and it is impossible to obtain an extremely strong perpendicular anisotropic magnetic field with the perpendicular magnetic recording medium comprising the Co-Cr film. Hence, there is another perpendicular magnetic recording medium in which a third element is added to Co-Cr so that the axis of easy magnetization of Co is satisfactorily perpendicular to the film. For example, niobium (Nb) or tantalum (Ta) is added to Co-Cr as the third element. In this case, due to the addition of Nb or Ta, the orientation of Co is improved so that the axis of easy magnetization is satisfactorily perpendicular to the film but the saturation magnetization of the perpendicular magnetic recording medium decreases by the addition of Cr and Nb (or Ta) which are non-magnetic materials to Co which is a ferromagnetic material. As a result, there is a disadvantage in that it is impossible to obtain a high reproduced output due to the saturation magnetization.
Accordingly, a perpendicular magnetic recording medium having a double film construction has been proposed. According to this perpendicular magnetic recording medium, a film having a high permeability, that is, a film having a low coercivity such as a nickel-iron (Ni-Fe) film, is formed between the Co-Cr film and the base film. The magnetic flux which is spread within the high permeability film is concentrated toward the magnetic pole of the perpendicular magnetic head at a predetermined magnetic recording position in order to obtain a strong magnetization which is in the perpendicular direction and does not spread in the longitudinal direction of the perpendicular magnetic recording medium. However, in the case of the perpendicular magnetic recording medium having the double film construction, the coercivity of the high permeability film is extremely small compared to the coercivity of the Co-Cr film, and there is a disadvantage in that Barkhausen noise is generated. For example, the coercivity of the Co-Cr film is over 700 Oe, and the coercivity of the high permeability film is under 10 Oe. In order to prevent the generation of the Barkhausen noise, the high permeability film must have a coercivity which is at least over 10 Oe, but there is no suitable material which satisfies such a condition and can also be used for the high permeability film.